i am interested to replace only cellName* with new cell names (say cellnameA cellNameB cellNameC cellNameD & cellNameE, for this i am using below onle liner but it doesnt work when the part of a cell name repeats multiple times it goes wrong.

If you use this one-liner, this cannot work, since the $existing_string and $replacing_string variables have not been defined anywhere in the script, so that the script does know what you want to match and by what you want the matched pattern to be replaced.

Actually, I don't know either what you want to replace by what. You don't give enough information for us to help you.

If you said something like "I want to replace 'cellName5' by 'Cell_name5'", I would immediately know what to suggest. But, here, we don't really know.

basically i want to find a reg-exp to search and replace a whole word, i use below option, but it doesnt work system("/usr/local/bin/perl -p -i -e 's/\b$existing_string\b/\b$replacing_string\b/g' /remote/de02h26/jmuddaia/tmp_esd_gold/USB_unique.Cat_copy");

If I understand you well, you're trying to launch a Perl one-line script from a Perl program, using the system function.

Why, for god's sake, would you want to to that? You can do directly in your Perl program everything that you want to do in the one-liner. Unless your favorite motto is: "why should we do things simple when they can be made complicated?"...